Californian dolphin gang caught killing porpoises

SEEMINGLY random acts of violence by bottlenose dolphins on porpoises could be down to sexual frustration among young males.

Cases of the cetaceans killing other creatures for no apparent reason have been reported in UK waters. Now bottlenose dolphins have been seen attacking harbour porpoises in the Pacific Ocean. Crucially, these observations show for the first time that the attackers are young males (Marine Mammal Science, DOI: 10.1111/j.1748-7692.2011.00474.x).

Mark Cotter at Okeanis, a non-profit conservation organisation in Moss Landing, California, and colleagues observed three acts of aggression by dolphins on lone porpoises. The dolphins chased the porpoises at high speed, rammed and then drowned them.

In one particularly violent attack, three dolphins corralled their victim before seven others joined them to ram the porpoise to death. Cotter found most shocking the fact that two dolphins remained behind to play with the carcass before pushing it towards his boat. "It was almost like they said: 'We're done playing with it, here you go'."

Competition for food does not seem to explain the attacks, as the dietary overlap between the two species is small, says Cotter. But the fact that 21 of the 23 attackers were males may be revealing. He believes that the attacks are "object oriented play" during the breeding season by young males who cannot get access to females because of competition from older males. "They are taking out their frustrations," he says.

"The identification of the 'culprits' as male is valuable," says Nick Tregenza, an adviser to the Marine Strandings Network in Cornwall, UK, and visiting researcher at the University of Exeter. That's because it helps narrow down potential explanations for the behaviour, he says.

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